Many systems for production of three-dimensional modeling by photohardening have been proposed. European patent application 250,121 filed by Scitex Corporation, Ltd. on Jun. 6, 1987, discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents pertinent to this art.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer. In addition to various exposure techniques, several methods of forming thin liquid layers are described which allow either the coating of a platform initially or the successive coating of object layers previously exposed.
U.S. Pat. No. 4,575,330 (C. W. Hull), issued on Mar. 11, 1986 and later reexamined (certificate issued on Dec. 19, 1989), describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction, wherein successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process. This patent also describes an embodiment in which a UV curable liquid floats on a heavier UV transparent liquid which is non-miscible and non-wetting with the curable liquid. In addition, this patent suggests the use of "water (or other) release coating" used in conjunction with a CRT and a fiber optic faceplate. Subsequent patent applications, made by Hull and his associates, published by the European Patent Office and listed in publication number 0 361 847 describe means of providing the thin layers of fluid more quickly using a doctor blade and of controlling the level in a vat of fluid.
U.S. Pat. Nos. 4,752,498 and 4,801,477 (E. V. Fudim) issued on Jun. 21, 1988 and Jan. 31, 1989 respectively, describe methods of forming three-dimensional objects, in which a sufficiently rigid transparent plate or film is placed in contact with a liquid photopolymer so as to hold the photopolymer surface to a desired shape, and preferably exclude air, during radiation curing through the transparent plate or film. It is further suggested that the surface of the transparent plate or film be made of a material which leaves the irradiated photopolymer surface capable of further crosslinking so that when a subsequent layer is formed it will adhere thereto. Fudim also suggests that this material be made of or contain in its molecules oxygen, copper or other inhibitors to aid in the release of the layer without distorting the solidified photopolymer.
Publication "Automatic Method for Fabricating a Three-Dimensional Plastic Model with Photohardening Polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, Nov. 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photo-forming polymer, of 2 mm thickness or less, to ultraviolet rays, and stacking the cross-sectional solidified layers. Publication "Solid Object Generation" by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, August 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a more recent publication, titled "A Review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15: 186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e. exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer. In addition to various exposure techniques, several methods of forming thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
Current methods of coating suggested thus far, however, have drawbacks in that they are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage to previously formed layers during the successive coating process. Furthermore, they omit to recognize very important parameters involved in the coating process such as, for example, the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as, for example, hydrogen bonds and substantially stronger forces such as, for example, mechanical bonds and vacuum or pressure differential forces on those thin layers and on the object being formed.
The Hull patent, for example describes a dipping process where a platform is lowered either one layer thickness or is dipped below the distance of one layer in a vat then brought up to within one layer thickness of the surface of the photohardenable liquid. Hull further suggests that low viscosity liquids are preferable, but for other practical reasons, the photohardenable liquids are generally high viscosity liquids. Although theoretically most liquids will flatten out due to surface tension effects, high viscosity liquids and even low viscosity liquids take an inordinate amount of time to flatten to an acceptable degree especially if large flat areas are being imaged and if the liquid layer thickness is very thin. Regions where previous layers consist of solid walls surrounding liquid pools further compounds the flattening process of the thin liquid layer coating. In addition, motion of the platform and parts, which have cantilevered or beam (regions unsupported in the Z direction by previous layer sections), within the liquid creates deflections in the layers contributing to a lack of tolerance in the finished object. In the embodiment where a heavier transparent liquid is utilized to create the thin flat layers of photopolymer that float on the transparent liquid, there is significant reliance on surface tension effects to ensure that the photopolymer layer will be flat. Reliance on these surface tension effects and the difference in specific gravities between the two liquids in order to create the flat photopolymer layers is severely complicated by other surface tension effects, such as, for example, meniscus development at the corners of the hardened photopolymer, and object geometries that create enclosed areas which produce substantial suction cup type lifting of the heavier liquid during coating of subsequent layers. In the embodiment where "water (or other) release coating" is proposed for use in conjunction with a CRT and a fiber optic faceplate, the patent does not teach methods by which a release coating could be applied and maintained on the faceplate surface.
The Muntz patent (U.S. Pat. No. 2,775,758 issued in 1956) and Scitex application describe methods by which the photohardenable liquid is introduced into the vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have all the problems of the Hull methods except that the deflections of the layers during coating is reduced.
The Fudim patent describes the use of a transmitting material, usually rigid and coated with a film or inherently unlikely to adhere to the hardened photopolymer, to fix the surface of the photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The methods described by Fudim do not address the problems inherent in separating such a transmissive material from a photopolymer formed in intimate contact with the surface of the transmissive material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings of inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage to the photopolymer during removal from the transmissive material surface. Furthermore, evaluations made by the Applicants indicate that the forces, resisting the separation or even sliding off the photohardenable material exposed in intimate contact with the suitably non-adhesive transmissive material, are capable of damaging the photoformed layer especially when surrounded by photohardenable liquid and even more especially when the photoformed layers are thin. No method is described in the Fudim patent to eliminate these problems.
In the Kodama (Kokai Patent No. SHO 56(1981)-144478, Japan, later published on Nov. 10, 1981) publication, mention is made of a Teflon, or polyethylene coated quartz plate, which coating acts as a releasing agent allowing the solidified resin to be easily removed from the base (quartz plate) and preferentially adhering to the constructing stand (aluminum sheet). This method would have all the difficulties mentioned in the Fudim reference above.
While it may be said that others such as Munz, Kodama, Cubital, Hull, etc. implicitly had air as the atmosphere at the interface of the photopolymer surface, air was not an element comprised within their specifications. And the presence, advantages, and uses of air in regions deeper into the compositions (also implicit in previous specifications) has not been specified though they are implicit.
In a thesis paper, published by the Department of Mechanical Engineering, University of Delaware, library catalogue date Aug. 14, 1990, there is mention of an unsuccessful effort, in which the author, Hirsch, studied the possibility of creating a porous fused silica plate through which the photopolymer could be exposed. The purpose of the porosity in the plate was to allow air to flow into the surface between the plate and the hardened layer to allow vacuum breaking when they were separated. It was also proposed that the air or oxygen passing through the plate might inhibit the polymerization at this surface aiding in release. This effort was unsuccessful primarily due to difficulties in obtaining a UV transparent porous plate material. But it also neglects concerns such as, for example, polymer adhesion to the fused silica in non-pore regions and eventual bridging of this polymer in subsequent coating applications which would close off the pores, the requirement for very small pore sizes which would severely restrict the air flow which is supposed to relieve the vacuum forces, and the lack of any driving forces or pressures to prevent the photopolymer from entering the pores or to push the air into the interface between the porous plate and the photopolymer surface.
One of the objects of the present invention is therefore to provide a method and apparatus for quickly producing layers of a liquid photoformable material, of preferably 0.030" thickness or less, which are flat to within preferably 0.001" per square inch or better, and by which previously exposed layers are minimally distorted or damaged during the coating process for the production of three-dimensional objects by sequential coating of said layers and exposure after each coating.